This renewal application addresses the role of the scaffolding protein Filamin A (FlnA) in the progression of prostate cancer (CaP), and tests the hypothesis that loss of FlnA nuclear localization in the luminal epithelial cells of CaP signifies resistance of the tumor to anti-androgens bicalutamide, abiraterone acetate or enzalutamide, that are used to treat patients with castration resistant prostate cancer (CRPC). This hypothesis is based on preliminary and published data demonstrating that in the presence of nuclear FlnA, CaP cells and animal models respond to anti-androgens whereas in the absence of nuclear FlnA, they do not respond. Based on available preliminary data, we propose that in the presence of nuclear FlnA, some AR transcriptional targets, such as TMPRSS2, VEGFA, GDF15, IL32, and JUN (which were identified in a RNA-Seq analysis as FlnA-regulated AR transcriptional targets), are selectively targeted for transcription only upon androgen stimulation. Since these genes regulate cell survival, this will cause androgen-addiction, whereby the cells become susceptible to apoptosis induction by anti-androgens. In Aim 1, we will identify targets of AR affected by FlnA nuclear localization. Since many of these are also targets of the transcriptional factor Sp1, we hypothesize that FlnA regulates the selectivity of AR targets by bringing AR together with selected transcriptional factors such as Sp1. Further, we hypothesize that FlnA regulates this selectivity and overall transcriptional activity by binding to the AR hinge region and thereby controlling the binding partners of AR. Therefore, nuclear FlnA can regulate androgen sensitivity by preventing binding to co-activators that encourage ligand-independent transcriptional activity. The significance of these studies is that they will determine whether in CRPC cells, AR loses its target specificity and that FlnA localization to the nucleus is able to restore that specificity. In Aim 2, we will investigate why FlnA is lost from the epithelia in some CRPC tumors and not in others. FlnA nuclear localization involves the cleavage of this molecule by calpains whereby one of the cleaved products translocates to the nucleus; however, phosphorylation of FlnA at S2152 impedes cleavage by these proteases, thereby preventing nuclear localization. Therefore, we will determine whether expression of FlnA in the nucleus or the lack thereof, results from a loss of equilibrium between calpain activity and FlnA phosphorylation. The hypothesis will be further tested using patient derived xenograft (PDX) tumor lines orthotopically implanted in the prostates of immunocompromised NOD-SCID IL-2R?-null (NSG) mouse, and will determine whether response to these drugs correlate with the expression of FlnA in the nuclei. Finally, in Aim 3, we will determine whether targets of FlnA-regulated AR transcriptional activity may be used to determine whether human patients with CRPC will respond to abiraterone or enzalutamide. This will be necessary because no tissue other than blood is usually available from CRPC patients for analysis, therefore, we will determine whether such markers exist in the serum or plasma, and whether these markers correlate with FlnA nuclear localization. Blood from 50 VA patients with CRPC, who will be treated with bicalutamide, abiraterone acetate+prednisone or with enzalutamide, as standard-of-care, will be used for these studies. The level of selected markers in the blood will be correlated with the response of these patients to the drugs. The significance of the proposed studies is that they will provide VA physicians with a tool to determine whether a patient will respond to the standard of care use of bicalutamide, abiraterone or enzalutamide. In addition, the proposed studies will determine the role of FlnA nuclear localization on AR transcriptional activity and confer a novel role for a co-regulator. Further, the genome wide analyses studies proposed will enable the identification of novel AR transcriptional targets.